Issue in Honor of Prof. Joan Bosch
ARKIVOC 2007 (iv) 344-355
Diels-Alder reaction between indoles and cyclohexadienes
photocatalyzed by a (thia)pyrylium salt
María González-Béjar,a Salah-Eddine Stiriba,a Miguel A. Miranda,b,*
and Julia Pérez-Prietoa,*
a
Departamento de Química Orgánica/Instituto de Ciencia Molecular, Universidad de Valencia,
Polígono La Coma, 46980, Paterna, Valencia, Spain
b
Departamento de Química/Instituto de Tecnología Química UPV-CSIC; Universidad
Politécnica de Valencia, Camino de Vera s/n, 46022, Valencia, Spain
E-mail: [email protected]
Dedicated to Prof. Joan Bosch on the occasion of his 60th anniversary
Abstract
Thiapyrylium salt 1 is an efficient photocatalyst in the Diels-Alder reaction between indoles
(InHs) and 1,3-cyclohexadienes (CHDs). For instance, irradiation of deaerated dichloromethane
solutions containing 1 (1 mM), indole (20 mM) and cyclohexadiene (40 mM), in the presence of
an acylating agent, led to the Diels-Alder cycloadduct in 62% yield (endo:exo ratio of 1.8:1).
Taking into account the very high intersystem crossing yield (Φisc = 0.97) of thiapyrylium salt 1,
involvement of its triplet excited state in this Diels-Alder reaction has been studied using steadystate and time-resolved experiments; the results are compared with those previously obtained for
pyrylium salts, which exhibit a considerable fluorescence quantum yield and are assumed to
operate via electron transfer from the InH to their singlet excited state.
Keywords: Diels-Alder
cyclohexadienes
reaction,
photocatalysis,
thiapyrylium
salts,
indoles,
1,3-
Introduction
The Diels-Alder reaction is a well established synthetic method in organic chemistry, which
allows creation of two new carbon-carbon bonds, leading to the formation of six-membered
rings. However, this reaction is generally inefficient when both diene (D) and dienophile (A) are
electron-rich compounds.1,2 Several strategies have been devised to overcome this limitation.3-7
One of them makes use of triarylpyrylium salts as electron transfer photocatalysts.6 Indole (5)
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quenches the fluorescence of triarylpyrylium salts; therefore, it has been suggested that the
photocatalysts singlet excited state is involved in the cycloaddition process.
Electrochemical and photophysical properties, such as reduction potential, fluorescence
quantum yield and T-T absorption spectrum of pyrylium salts, are often influenced by the
substitution pattern of the aryl groups and by the nature of the heteroatom in the heterocycle.8
The value of the oxidation potential seems to be a major factor conditioning the product yields
and product distribution.8 In particular, it has been published6 that the Diels-Alder reaction
between phellandrene (4b) and 5 improves from 57% to 67% when tris(4methoxyphenyl)pyrylium 3 (Ered = -0.52 V vs SCE) is used instead of 2 (Ered = -0.29 V vs SCE)9
as catalyst (Chart 1, Scheme 1). Also, the fluorescence quantum yield of 3 (Φf = 0.97), the
highest among the studied pyrylium salts, could contribute to the increased reaction efficiency.
Concerning the effect of spin multiplicity of the photocatalyst on this cycloaddition, no data has
been reported as yet. Taking into account that the sulfur atom in thiapyrylium salt 1 exerts an
internal heavy atom effect, leading to a very high intersystem crossing quantum yield (Φisc =
0.97),10 it appeared of interest to study the role of this factor in the cycloaddition between InHs
and 1,3-cyclohexadienes (CHDs). Herein, we report that, as a matter of fact, 1 photocatalyzes
this Diels-Alder reaction with similar efficiency as the oxa-analogues, though with a lower
stereoselectivity. Steady-state and time resolved studies agree well with the involvement of both
the singlet and the triplet excited states.
R
R
X
Y
R
1 R = H, X = S, Y = ClO4
2 R = H, X = O, Y = BF4
3 R = OCH3, X = O, Y = BF4
Chart 1. Structure of the photocatalysts.
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R1
R3
H
N
+
R2
4a R = R = H
4b R1 = CH3, R2 = CH(CH3)2
1
5 R3 = H
6 R3 = CH3
2
CH3COCl
S*
NaHCO3
R1
R1
H
2
R
H
N
C
O
H
2
+ R
H
R3
CH3
O
endo
1
2
N
C
R3
CH3
exo
3
7a R = R = R = H
7b R1 = CH3,R2 = CH(CH3)2, R3 = H
8 R1 = R2 = H, R3 = CH3
Scheme 1. Diels-Alder reaction photocatalyzed by pyrylium salts.
Results
Steady-state irradiation
The UV-visible absorption spectra of 1 in the absence and in the presence of indole (5) and CHD
(4a) were obtained in dichloromethane. The absence of any characteristic charge transfer (CT)
absorption in the presence of the quenchers agreed with the absence of ground-state
complexation (Figure 1).10 The intense absorption band at λ>320 nm allows selective excitation
of the thiapyrylium salt in the presence of InHs and CHDs.
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3.0
Absorbance
2.5
2.0
1.5
1.0
0.5
0.0
300
400
500
Wavelength, nm
600
….
Figure 1. Absorption spectra of 1 (1.6 x 10-5 M) in the absence (—) and in the presence of 5 ( )
(3,2 x 10-4 M) or 4a (---) ( 6,4 x 10-4 M) in dichloromethane.
Product studies were performed by irradiation (UVA lamp emitting at 400 nm ≥ λ ≥ 320
nm) of deaerated dichloromethane solutions of 1 (0.001 M), 4a (0.04 M) and 5 (0.02 M) for 6 h.
Previous reports on this reaction using triarylpyrylium salts as photocatalysts have stressed the
need to protect the amino group of the nascent photoadduct.6 Therefore, an acylation reagent
(acetyl chloride/NaHCO3) was also added to the reaction mixture prior to irradiation.
Indeed, the acylated Diels-Alder products 7a were obtained in ca. 62 % yield, with an
endo:exo ratio close to 1.8:1 (Scheme 1 and run 1 of Table 1). Slightly higher yield (70%) and
selectivity (endo:exo 3.3/1.0) has been achieved using 2 as photocatalyst (run 2, Table 1).6 As in
the case of using pyrylium salts 2 or 3 as photosensitizers, no [2+2] cross-cycloaddition products
were detected.6
Electron transfer from 4a to 1 led to CHD dimers as by-products (7 % relative to the initial
amount of 1,3-cyclohexadiene), with an isomer distribution typical of radical cation
dimerizations. The major product was the endo-[4+2] dimer, which agrees with the high
selectivity found when 1 was used for the dimerization of CHD in dichloromethane.8
Control experiments showed that [4+2] cross-cycloaddition did not occur in the absence of 1.
Also, irradiation of N-acetylindole and cyclohexadiene in the presence of 1 gave only traces
amounts of 7a.
α-Phellandrene (racemic mixture) 4b was also tested in the cross-cycloaddition with 5
photoinduced by 1. This 2,5-disubstituted CHD derivative led to a similar yield of Diels-Alder
products as 4a (compare runs 1 and 3 of Table 1). However, 2 was less efficient with 4b than 4a,
and formation of 7b occurred with a 57% yield.6 Better efficiency and selectivity were achieved
with the stronger electron acceptor pyrylium salt 36 (run 5 of Table 1), which also has a higher
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fluorescence quantum yield (Φf = 0.97 for 3 vs 0.47 for 2). The relative stereoselectivity found in
the pyrylium salts-promoted cycloaddition follows the order 1 < 2 < 3.
Table 1. Diels-Alder reaction between indoles and cyclohexadienes photocatalyzed by pyrylium
salts in dichloromethane.
Run
Cat.
Diene
Indole
1
2
3
4
5
6
1
2
1
2
3
1
4a
4a
4b
4b
4b
4a
5
5
5
5
5
6
Diels-Alder
Yield (%)a
7a (62)
7a (70) c
7b (66)
7b (57)c
7b (67) c
8 (33)
endo:exob
1.8:1.0
3.3:1.0
1.7:1.0
2.4:1.0
4.6:1.0
1.9:1.0
a
Spectroscopically determined based on the initial amount of indole, no other products derived
from indole were detected. If calculations are done based on initial CHD, the yields are halved. b
Determined by comparing the integrals of the characteristic 1H-NMR signals between 4.2 and
4.8 ppm for the two rotamers of each stereoisomer. c reference 6.
On the other hand, experiments previously performed with 2 have shown that 2-methylindole
does not lead to photoadducts with CHD in the pyrylium salt-photocatalyzed process, while
methyl 3-indolylacetate reacts only sluggishly.6 These results have been attributed to steric
hindrance at the attacked indole positions. In order to check the possible influence of steric
effects at other positions, 7-methylindole (6) was used as dienophile with 1 as photocatalyst; this
provided 811 in a moderate yield and similar stereoselectivity as 7a (run 6, Table 1).
Fluorescence studies
To get some insight into the photophysical behavior of the excited singlet state of 1, its
quenching by 5 and 4a was examined in dichloromethane. Indeed, the fluorescence (λmax = 464
nm in dichloromethane, Figure 2) of 1 was efficiently quenched by both types of substrates. The
fluorescence quenching rate constants by indole (1kq = 2.2 x 1010 M-1s-1) and cyclohexadiene (1kq´
10
-1 -1
= 0.7 x 10 M s ) were obtained from the Stern-Volmer analysis (eqs 1-4, Figure 3), using the
singlet lifetime of 1 τf = 3.6 ns. 8
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Ι0 / Ι1 = 1 + KSV[InH] (1)
KSV = τf1kq (2)
Ι0 / Ι2 = 1 + KSV´[CHD] (3)
KSV´ = τf1kq´ (4)
Normalized fluorescence
1.00
0.75
0.50
0.25
0.00
250
300
350
400
450
500
550
600
Wavelength, nm
Figure 2. Normalized fluorescence emission (right) and excitation spectra (left) of 1 (4,7 x 10-5
M) in dichloromethane.
The free energy change associated with electron transfer from indole to the excited singlet
state of the thiapyrylium salt was estimated according to the Rehm-Weller equation (eq 5)
(5)
∆G0ET = Eox(D/D+)- Ered(A-/A) - E0,0* + 2.6 eV/ε – 0.13 eV
-1 12
where E0,0* is the singlet energy of the thiapyrylium salt (66.0 kcal.mol ).
Thus, the free energy change, ∆G0ET, for the electron transfer process in dichloromethane
was estimated as -32.1 kcal.mol-1 using the known half-wave oxidation potential of indole
(around 1.1 V vs SCE)13 and half-wave reduction potential of 1 (-2.53 V vs SCE ).14
The rate constants for quenching of the fluorescence of pyrylium salt 2 by indole and
cyclohexadiene were also determined (5.2 x 1010 M-1s-1 and 1.0 x 1010 M-1s-1, respectively). In
addition, the free energy change associated with electron transfer from 5 to the excited singlet
state of the pyrylium salt was estimated at -30.2 kcal.mol-1.
Intersystem crossing contribution to deactivation of the thiapyrylium singlet
To quantify the amount of triplet excited state of 1 formed, the relative intersystem crossing
yields under the employed reaction conditions were estimated taking into account the main
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deactivation pathways of the singlet excited state: fluorescence emission, fluorescence
quenching, and intersystem crossing. Competition between them should be governed by the
respective rate constants and by the quencher concentration.
The rate constant of fluorescence emission (kf) and intersystem crossing (kISC) are intrinsic
properties of the photocatalyst. The values for 1 were calculated from eqs 6 and 7 (τf = 3.6 ns8
and Φf = 0.0315) and found to be kf = 0.8 x 107 s-1 and kISC =2.7 x 108 s-1.
τfkf = Φf (6)
τfkISC = ΦISC (7)
0.7
A
0.6
(I0/I)-1
0.5
0.4
0.3
0.2
0.1
0
0
1
2
3
4
5
6
7
8
9
10
[4a] (mM)
0.25
B
(I0/I)-1
0.20
0.15
0.10
0.05
0
0
0.5
1
1.5
[5] (mM)
2
2.5
3
Figure 3. Stern-Volmer plots for fluorescence quenching of 1 ( 4.7 x 10-5 M) by A: 4a and B: 5
in dichloromethane.
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From these data and the rate constants for quenching of the thiapyrylium salt fluorescence,
the contributions of the different singlet deactivation pathways can be estimated (eqs 8-12).
ks = kf + kISC +1kq [InH] + 1kq´ [CHD] (8)
Residual Fluorescence: kf / ks = 0.8 % (9)
Quenching of fluorescence by 5: 1kq[InH] / ks = 44.0 % (10)
Quenching of fluorescence by 4a: 1kq´[CHD] / ks = 28.0 % (11)
Intersystem crossing: kISC / ks = 27.0 % (12)
Thus, it is clear that i) intersystem crossing to the triplet excited state has a considerable
contribution to singlet deactivation, and ii) the triplet excited state of the photocatalyst is
sufficiently populated. It may behave as electron acceptor towards indole, since this process is
thermodynamically feasible. Accordingly, the free energy change for the electron transfer
process in dichloromethane was estimated as ∆G0ET= _18.1 kcal.mol-1 using eq. 5, where E0,0* is
the triplet energy of the thiapyrylium salt (52.0 kcal.mol-1).14
Similar calculations with 2 led to a relative intersystem crossing yield of 8%.
Quenching of the photocatalyst triplet
Laser excitation (Nd:YAG, 10 ns laser pulse, λexc = 355 nm) of dichloromethane solutions
containing thiapyrylium salt 1 (1.6 x 10-5 M) gave rise to a depletion of the ground state
absorption between 390 and 440 nm. This was followed by the appearance of a broad transient
absorption between 460 and 600 nm, with a lifetime of ca. 6.9 µs (Figure 4), which was assigned
to the known T-T transition of 1.15
Both diene 4a and dienophile 5 quenched the triplet excited state of the thiapyrylium salt; the
rate constants for triplet quenching (3kq and 3kq´) were measured by monitoring the decay of the
T-T transition at 500 nm, using increasing amounts of the quencher and recording the SternVolmer plots (Figure 5). Thus, values of 8.8 x 109 M-1s-1 and 4.7 x 108 M-1s-1 were calculated for
5 and 4a, respectively.
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Absorbance
0.04
0.02
0.00
-0.02
-0.04
300 350 400 450 500 550 600 650 700 750
Wavelength, nm
Figure 4. Transient absorption spectra recorded following laser excitation (355 nm) of 1 (1.6 x
10-5 M) in deaerated dichloromethane 3 µs after the laser pulse.
A
(τ0/τ)-1
1.2
1.0
0.8
0.6
0.4
0.2
0
0
0.05
0.1
0.15
[4a] (mM)
B
2.5
(τ0/τ)-1
2
1.5
1
0.5
0
0
0.005
0.01
0.015
0.02
[5] (mM)
Figure 5. Stern-Volmer plots for triplet quenching of 1 (1.6 x 10-5 M) by A: 4a and B: 5 in
dichloromethane.
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Discussion
Photoinduced electron transfer (PET) processes involve exchange of one electron between the
excited state of the photocatalyst (singlet or triplet) and the quencher, generating a radical ion
pair. The efficiency of the subsequent processes can depend on spin multiplicity.16 Triplets are
usually longer-lived, which allows the partners to diffuse and react. Moreover, back electron
transfer is slow within the triplet radical ion pairs, facilitating cage escape. Pyrylium salts are
well-established PET photocatalysts; depending on the substitution pattern they can generate
singlet or triplet radical ions pairs. The use of thiapyrylium salt 1, with the highest intersystem
crossing quantum yield, allows determining the effect of the triplet on the Diels-Alder reaction of
InHs with CHDs.
The fluorescence quenching of 1 by indole and the large free energy changes associated with
electron transfer from indole to the excited singlet state of the thiapyrylium salt strongly suggest
that this process takes place quite efficiently from 5 to the singlet excited state of 1.
Nevertheless, the triplet is also populated and efficiently quenched by 5, being electron transfer
thermodynamically feasible. Therefore, the electron transfer from indole not only to the singlet
excited state of 1 but also to the triplet can be an important primary process.
The fact that the yields of the Diels-Alder reaction are similar for 1 and 2, but with reduced
stereoselectivity when using the former, could agree with the much higher contribution of the
photocatalyst triplet excited state in the case of the thiapyrylium salt.
Conclusions
The [4+2]-cycloaddition of indoles with 1,3-cyclohexadienes can be efficiently photocatalyzed
by thiapyrylium salts. The experimental results agree with partial involvement of the triplet
excited state in the electron transfer process. Cross-cycloadducts yields are similar to those
obtained using the oxa-analogues, though the process is less stereoselective. So far, this is the
only reported example of thiapyrylium photocatalyzed cycloaddition of indoles with
cyclohexadienes.
Experimental Section
General Procedures. 1H and 13C NMR spectra were recorded in a (400 MHz) Brucker
spectrometer; chemical shifts (δ) are reported in ppm relative to TMS. The coupling constants (J)
are in hertz (Hz). Dichloromethane was dried over CaCl2 and distilled over CaH2 prior use. All
reagents were commercially available and used without further purification, except 1,3cyclohexadiene (distilled over NaBH4). Column chromatography was performed on silica gel 60
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(0.040-0.063 mm). GC/MS data are taken under electron ionization (EI) conditions of about 30
eV. High-resolution mass spectra (HRMS) were conducted at the SCSIE in Valencia, Spain.
Steady-state spectroscopic measurements. UV/vis absorption measurements were performed
on a Agilent 8453 spectrometer. Fluorescence emission and excitation spectra were recorded on
a Photon Technology International (PTI) LPS-220B fluorimeter. Measurements were done at 25
°C in cuvettes of 1 cm path length.
Laser flash photolysis. Laser flash photolysis experiments were carried out by using the third
harmonics (355 nm) of a pulsed Nd:YAG laser. The pulse duration was 10 ns, and the energy of
the laser beam was 18 mJ /pulse. A xenon lamp was employed as detecting light source. The
laser apparatus consisted of the pulsed laser, the Xe lamp, a monochromator, a photomultiplier
(PMT) system and an oscilloscope. The output signal was transferred to a personal computer for
data analysis.
Standard procedure for the Diels-Alder reactions. To a deaerated dichloromethane (5 mL)
solution of indole (0.02 M), 1,3-cyclohexadiene (0.04 M) and powdered NaHCO3 (18 mg),
acetyl chloride (0.02 M) was added. Then, the thiopyrylium salt (0.001 M) was added under a
nitrogen atmosphere. Irradiation was carried out in a photoreactor equipped with eigth UVA
lamps (400 nm ≥λ ≥ 320 nm, λmax centered at 350 nm). The solutions were irradiated for 6 h,
then filtered, washed with water and dried over Na2SO4. The solvent was evaporated and the
crude was analyzed by 1H-NMR.
Acknowledgements
We thank the Ministerio de Educación y Ciencia (Project CTQ2005-00569, doctoral fellowship
to M.G.B.) and Generalitat Valenciana (GV-ACOMP06/134) for generous support of this work.
References and Footnotes
1.
2.
3.
(a) An interesting exception is the Diels-Alder reaction between C60 and Danishefsky´s
dienes: Mikami, K.; Matsumoto, S.; Okubo, Y.; Fujitsuka, M.; Ito, O.; Suenobu, T.;
Fukuzumi, S. J. Am. Chem. Soc. 2000, 122, 2236. (b) Strained dienophiles can also lead to
[4+2] adducts: Collins, S. K.; Yap, G. P. A.; Fallis, A. G. Org. Lett. 2002, 4, 11.
Some thermal Diels-Alder reactions between electron-rich components are metal-catalyzed
processes: Hilt, G.; Smolko, K. I. Synthesis 2002, 686.
(a) Mattay, J.; Trampe, G.; Runsink, J. Chem. Ber. 1988, 121, 1991. (b) Mattay, J.;
Vondenhof, M.; Denig, R. Chem. Ber. 1989, 122, 951.
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4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
ARKIVOC 2007 (iv) 344-355
(a) Pabon, R. A.; Bellvile, D. J.; Bauld, N. L. J. Am. Chem. Soc. 1983, 105, 5158. (b)
Schmittel, M.; von Seggern, H. J. Am. Chem. Soc. 1993, 115, 2165. (c) Bauld, N. L.; Yang,
J.; Gao, D. J. Chem.Soc., Perkin Trans. 2 2000, 207.
(a) Calhoun, G. C.; Schuster, G. B. Tetrahedron Lett. 1986, 27, 911. (b) Akbulut, N.;
Hartsough, D.; Kim, J. I.; Schuster, G. B. J. Org. Chem. 1989, 54, 2549. (c) Kim, J. I.;
Schuster, G. B. J. Am. Chem. Soc. 1992, 114, 9309.
(a) Gieseler, A.; Steckhan, E.; Wiest, O.; Knoch, F. J. Org. Chem. 1991, 56, 1405. (b) Wiest,
O.; Steckhan, E. Tetrahedron Lett. 1993, 34, 6391.
(a) Pérez-Prieto, J.; Stiriba, S.-E.; González-Béjar, M.; Domingo, L. R.; Miranda, M. A.
Org. Lett. 2004, 6, 3905. (b) González-Béjar, M.; Stiriba, S.-E.; Domingo, L. R.; PérezPrieto, J.; Miranda, M. A. J. Org. Chem. 2006, 71, 6932.
Martiny, M.; Steckhan, E.; Esch, T. Chem. Ber. 1993, 126, 1671.
Saeva, F. D.; Olin, G. R. J. Am. Chem. Soc. 1980, 102, 299.
Jayanthi, S. S.; Ramamurthy, P. J. Phys. Chem. A 1998, 102, 511.
Characterized by comparison with analogous cycloadducts. Endo-8: 1H NMR (400 MHz,
CDCl3): δ 0.9-1.8 (m, 4H), 2.2 (s, 3H), 2.3 (s, 3H), 2.9 (m, 1H), 3.0 (m, 1H), 3.8 (dd, J (H,
H) = 8.8 , 3.2 Hz, 1H), 4.4 (d, J(H, H) = 7.6 Hz, 1H), 5.7 (t, J (H, H) = 7.4 Hz, 2H), 6.0 (t, J
(H, H) = 7.4 Hz, 2H), 6.9-7.0 (m, 3H); MS (30 eV): m/z (%): 253 (12) [M+], 173 (44), 131
(100); FAB-MS calcd for C17H20NO: [M++1], 254.1545. Found: 254.1544. Exo-8: relevant
1
H NMR (400 MHz, CDCl3) signals: δ 2.8 (m, 1H), 2.9 (m, 1H), 3.6 (m, 1H), 4.3 (d, J (H,
H) = 7.6 Hz, 1H); MS (30 eV): m/z (%): 253 (9) [M+], 173 (53), 131 (100).
Miranda, M. A.; Izquierdo, M. A.; Pérez-Ruiz, R. J. Phys. Chem. A 2003, 107, 2478.
Pérez-Prieto, J.; Bosca, F.; Galian, R. E.; Lahoz, A.; Domingo, L. R.; Miranda, M. A. J.
Org. Chem. 2003, 68, 5104.
Miranda, M. A.; García, H. Chem. Rev. 1994, 94, 1063.
Akaba, R.; Kamata, M.; Koike, A.; Mogi, K-I.; Kuriyama, Y.; Sakuragi, H. J. Phys. Org.
Chem. A 1997, 10, 861.
Julliard, M.; Chanon, M. Chem. Rev. 1983, 83, 425.
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